1. Field of the Invention
The present invention relates to a method of manufacturing an oxide superconductor.
2. Description of the Prior Art
A superconductive material composed of YBa.sub.2 Cu.sub.3 O.sub.7-.delta., which shows superconductivity at a high temperature of about 90.degree. K, is expected for various uses. For example, such a superconductive material can be applied to power transmission, in order to extremely reduce power loss of about 7% caused by transmission. With respect to a use as an electromagnet, studied is application to nuclear fusion, SOR, an accelerator, a magnetic levitation train, NMR-CT, an electromagnetically driven ship or the like.
In application to such usage, it is preferable to work the superconductive material having the aforementioned composition into the form of a bulk material, a wire rod, a tape or the like. In this case, a value generally obtained as critical current density Jc is about 10.sup.3 to 10.sup.4 A/cm.sup.2 at the liquid nitrogen temperature of 77.degree. K. However, it is said that critical current density Jc of at least 10.sup.6 A/cm.sup.2 is required for practical use such as power transmission or application to an electromagnet.
A bulk oxide superconductor has been mainly manufactured by a solid phase reaction method or a coprecipitation method.
Powder of the aforementioned yttrium oxide superconductive material is generally synthesized by the solid phase reaction method.
A YBa.sub.2 Cu.sub.3 O.sub.7-.delta. material is prepared by the sintering method as follows:
First, BaCO.sub.3, Y.sub.2 O.sub.3 and CuO are weighted as starting materials in the ratios Y:Ba:Cu=1:2:3, and mixed in a mortar. Powder of such a mixture is heated/reacted within an alumina crucible in an oxygen flow or in the atmosphere at 930.degree. to 950.degree. C. for 8 to 12 hours. Then the powder is ground and pelletized, which is sintered in the air or in an oxygen flow of 900.degree. to 950.degree. C. for 12 to 24 hours, and then subjected to furnace cooling.
In the coprecipitation method, reagents such as oxalate, carbonate and citrate are employed to synthesize YBa.sub.2 Cu.sub.3 O.sub.7-.delta. through thermal decomposition of such reagents.
Another oxide superconductor expressed as MBa.sub.2 Cu.sub.3 O.sub.7-.delta., where M represents at least a single element selected from a group of Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, is also prepared by a substantially similar synthesizing method.
A vapor phase growth method has been conventionally reported as a method of preparing a thin film of an oxide superconductor. For example, there are reports that an oxide thin film has been formed by molecular beam epitaxy (MBE) to obtain a thin film having b and c axes oriented in surfaces, that a thin film of 1 .mu.m in thickness has been grown on an SrTiO.sub.3 substrate by a three-gun vapor deposition method, and the like.
In the conventional solid phase reaction method, powder materials are mixed with each other in a mortar. However, the degree of such mixing is to about the size of the starting particles at the most, and homogenous dispersion is limited. Further, since solid phase reaction progresses only in the vicinity of contact points between particles, such reaction must be repeated in order to obtain a YBa.sub.2 Cu.sub.3 O.sub.7-.delta. phase. In addition, the obtained sample is porous and its density is merely about 70 to 80% of a theoretical value.
In a conventional method of manufacturing an oxide superconductor by the vapor phase growth method, on the other hand, manufacturing steps are complicated and the growth rate of a film is slow, leading to inferior productivity. In the conventional method, further, such a problem is particularly caused in density that a body obtained is considerably porous with density similar to that of a sintered material, and a critical current value is reduced as the result.